Background: Short-term structured risk assessment is presumed to reduce incidents of aggression and seclusion on acute psychiatric wards. Controlled studies of this approach are scarce. Aims: To evaluate the effect of risk assessment on the number of aggression incidents and time in seclusion for patients admitted to acute psychiatric wards.Method: A cluster randomised controlled trial was conducted in four wards over a 40-week period (n = 597 patients). Structured risk assessment scales were used on two experimental wards, and the numbers of incidents of aggression and seclusion were compared with two control wards where assessment was based purely on clinical judgement. Results: The numbers of aggressive incidents (relative risk reduction 768%, P<0.001) and of patients engaging in aggression (relative risk reduction RRR =-50%, P<0.05) and the time spent in seclusion (RRR =745%, P<0.05) were significantly lower in the experimental wards than in the control wards. Neither the number of seclusions nor the number of patients exposed to seclusion decreased. Conclusions: Routine application of structured risk assessment measures might help reduce incidents of aggression and use of restraint and seclusion in psychiatric wards.